Re: [JBoss-user] Fundamental error

[Timo K. <ko...@jy...>]

Hi.

I am rookie with JBOSS. But could it be possible that you are using "Instance 
Per Transaction Policy" with your CMP beans ?

Read the <container-configurations> part in 
$JBOSS_HOME/server/default/conf/standardjboss.xml and check what your 
application's jboss.xml says about  container-configuration.

--
Timo

On Thursday 17 April 2003 12:33, Brian McSweeney wrote:
> Hi Jos,
>
> thanks for the reply. I really want to get this solved. The first step in
> understanding why it is happening is to clear up what transactions
> should be doing.
>
> > A transaction does not perform locking in order to guarantee mutually
> > exclusive access to a row.
>
> Is this definately true? I quote from the JBoss Admin developer guide,
> p181:
>
> "A transaction lock ensures that only one transaction at a time has
> access to a give Entity Bean. This ensures the ACID properties
> of transactions at the application server level. Since, by default,
> there is only one active instance of any given Entity Bean at one time,
> JBoss must protect this instance from dirty reads and dirty writes.
> So, the default Entity Bean locking behavior will lock an Entity Bean
> within a transaction until it completes. This means that if any method
> at all is invoked on an Entity Bean within a transaction,
> no other transaction can have access to this bean until the
> holding trans-action commits or is rolled back. "
>
> again this is repeated on page 182:
>
> "The default locking policy of JBoss is to lock an Entity bean when an
> invocation occurs in the context of a transaction until the transaction
> completes."
>
> Now I know that this might be obvious to some people, but can
> somebody please give me a definative explanation of what
> transactions actually lock using JBoss!!!?????
>
> This is fundamental to what JBoss is used for and surely someone
> can tell me.
> thanks,
> Brian
>
>
> ----- Original Message -----
> From: "Jos Visser" <jo...@os...>
> To: <jbo...@li...>
> Sent: Thursday, April 17, 2003 9:29 AM
> Subject: Re: [JBoss-user] Fundamental error
>
> > Hi,
> >
> > As far as I can see, this has nothing to do with transactions, but more
> > with the fact that lots of people misunderstand transactions.
> >
> > The implementation of the Sequence Pattern that lots of people use is
> > fundamentally broken, because it does not withstand two threads calling
> > nextValue() each in their own transaction from returning the same
> > (supposedly unique, but in fact not) value.
> >
> > See also the interesting thread on this at:
> >
> > http://www.theserverside.com/patterns/thread.jsp?thread_id=220
> >
> > A transaction does not perform locking in order to guarantee mutually
> > exclusive access to a row.
> >
> > ++Jos.nl
> >
> > And thus it came to pass that Brian McSweeney wrote:
> > (on Mon, Apr 07, 2003 at 05:26:25PM +0100 to be exact)
> >
> > > Hi all,
> > >
> > > we have a fundamental problem with primary key generation with JBoss
>
> using
>
> > > the
> > > xpetstore UIDGenerator component. This component has two classes. A
>
> session
>
> > > facade UIDGeneratorEJB which has a method getUniqueId, which calls a
> > > nextValue
> > > method on an entity bean ( CounterEJB ). This is an implementation of
>
> Floyd
>
> > > Marinescu's
> > > Sequence pattern.
> > >
> > > The UIDGeneratorEJB is marked transaction required. However a simple
>
> test
>
> > > shows that
> > > several threads are able to similtaniously call nextValue and get back
>
> the
>
> > > same integer.
> > > Something is surely critically wrong with our implementation or with
>
> JBoss.
>
> > > This scenario happens on JBoss 3.x with MySql.
> > >
> > > The two xpetstore files are below with a simple test class.
> > >
> > > Any help at all would be very much appreciated.
> > >
> > > Brian
> > >
> > > file://-------------
> > >
> > > package xpetstore.util.uidgen.ejb;
> > >
> > > import javax.ejb.EJBException;
> > > import javax.ejb.FinderException;
> > > import javax.ejb.SessionBean;
> > >
> > > import xpetstore.util.uidgen.interfaces.CounterLocal;
> > > import xpetstore.util.uidgen.interfaces.CounterLocalHome;
> > > import xpetstore.util.uidgen.util.CounterUtil;
> > >
> > >
> > > /**
> > >  * @author <a href="mailto:tch...@us...">Herve
> > > Tchepannou</a>
> > >  *
> > >  * @ejb.bean
> > >  *         name="UIDGenerator"
> > >  *         type="Stateless"
> > >  *          local-jndi-name="local/xpetstore.util.uidgen.UIDGenerator"
> > >  *         view-type="local"
> > >  * @ejb.transaction
> > >  *      type="Required"
> > >  * @ejb.util
> > >  *      generate="physical"
> > >  */
> > > public abstract class UIDGeneratorEJB
> > >     implements SessionBean
> > > {
> > >     file://~
> > > Methods
> > > ----------------------------------------------------------------
> > >
> > >     file://=================================
> > >     // Business Methods
> > >     file://=================================
> > >
> > >     /**
> > >      * @ejb.interface-method
> > >      */
> > >     public int getUniqueId( String idPrefix )
> > >     {
> > >         return getCounter( idPrefix ).nextValue(  );
> > >     }
> > >
> > >     file://=================================
> > >     // Misc Method
> > >     file://=================================
> > >     private CounterLocal getCounter( String name )
> > >     {
> > >         try
> > >         {
> > >             CounterLocal     counter = null;
> > >             CounterLocalHome home = CounterUtil.getLocalHome(  );
> > >
> > >             try
> > >             {
> > >                 counter = home.findByPrimaryKey( name );
> > >             }
> > >             catch ( FinderException fe )
> > >             {
> > >                 counter = home.create( name );
> > >             }
> > >
> > >             return counter;
> > >         }
> > >         catch ( Exception ce )
> > >         {
> > >             throw new EJBException( "Could not create counter " + name
> > > +
>
> ".
>
> > > Error: " + ce.getMessage(  ) );
> > >         }
> > >     }
> > > }
> > >
> > >
> > >
> > > package xpetstore.util.uidgen.ejb;
> > >
> > > import javax.ejb.CreateException;
> > > import javax.ejb.EntityBean;
> > >
> > >
> > > /**
> > >  * @author <a href="mailto:tch...@us...">Herve
> > > Tchepannou</a>
> > >  *
> > >  * @ejb.bean
> > >  *      name="Counter"
> > >  *      type="CMP"
> > >  *      view-type="local"
> > >  *      local-jndi-name="local/xpetstore.util.uidgen.Counter"
> > >  *      primkey-field="name"
> > >  *      schema="Counter"
> > >  *      cmp-version="${ejb.cmp.version}"
> > >  * @ejb.transaction
> > >  *      type="Required"
> > >  * @ejb.util
> > >  *      generate="physical"
> > >  * @ejb.persistence
> > >  *      table-name="T_COUNTER"
> > >  *
> > >  * @jboss.persistence
> > >  *      create-table="${jboss.create.table}"
> > >  *      remove-table="${jboss.remove.table}"
> > >  */
> > > public abstract class CounterEJB
> > >     implements EntityBean
> > > {
> > >     file://~
> > > Methods
> > > ----------------------------------------------------------------
> > >
> > >     file://=================================
> > >     // Business methods
> > >     file://=================================
> > >
> > >     /**
> > >      * @ejb.interface-method
> > >      */
> > >     public int nextValue(  )
> > >     {
> > >         int value = getValue(  ) + 1;
> > >         setValue( value );
> > >
> > >         return value;
> > >     }
> > >
> > >     file://========================================
> > >     // CMP fields
> > >     file://========================================
> > >
> > >     /**
> > >      * @ejb.pk-field
> > >      * @ejb.persistence
> > >      *      column-name="cnt_name"
> > >      *      jdbc-type="VARCHAR"
> > >      *      sql-type="varchar(50)"
> > >      * @ejb.interface-method
> > >      * @ejb.transaction
> > >      *      type="NotSupported"
> > >      */
> > >     public abstract String getName(  );
> > >
> > >     public abstract void setName( String name );
> > >
> > >     /**
> > >      * @ejb.persistence
> > >      *      column-name="cnt_value"
> > >      */
> > >     public abstract int getValue(  );
> > >
> > >     public abstract void setValue( int value );
> > >
> > >     file://========================================
> > >     // EJB callbacks
> > >     file://========================================
> > >
> > >     /**
> > >      * @ejb.create-method
> > >      */
> > >     public String ejbCreate( String name )
> > >         throws CreateException
> > >     {
> > >         setName( name );
> > >         setValue( 0 );
> > >
> > >         return null;
> > >     }
> > >
> > >     public void ejbPostCreate( String name )
> > >         throws CreateException {}
> > > }
> > >
> > >
> > >
> > > package xpetstore.util.uidgen.test;
> > >
> > > import javax.ejb.CreateException;
> > >
> > > import javax.naming.NamingException;
> > >
> > > import junit.framework.*;
> > >
> > > import xpetstore.util.uidgen.util.UIDGeneratorUtil;
> > >
> > >
> > > public class UIDGenTest
> > >     extends TestCase
> > > {
> > >     file://~ Static
> > > fields/initializers ---------------------------------------------
> > >
> > >     private static String COUNTER = "Test";
> > >
> > >     file://~
> > > Constructors
> > > -----------------------------------------------------------
> > >
> > >     public UIDGenTest( String name )
> > >     {
> > >         super( name );
> > >     }
> > >
> > >     file://~
> > > Methods
> > > ----------------------------------------------------------------
> > >
> > >     public void testCounter(  )
> > >         throws Exception
> > >     {
> > >         System.out.println( "Testing counter creation" );
> > >
> > >         for ( int i = 0; i < 10; i++ )
> > >         {
> > >             Thread t = new CounterThread( "Thread " + i );
> > >             t.start(  );
> > >         }
> > >     }
> > >
> > >     private void count( String name )
> > >         throws CreateException
> > >     {
> > >         int uid;
> > >         try
> > >         {
> > >             uid =
> > > neratorUtil.getLocalHome(  ).create(  ).getUniqueId( COUNTER );
> > >         }
> > >         catch ( NamingException n )
> > >         {
> > >             throw new CreateException( "Unable to generate the UId.
>
> Error="
>
> > > + n.toString(  ) );
> > >         }
> > >
> > >         System.out.println( "The counter created is: " + uid + ",
>
> created
>
> > > by: " + name );
> > >     }
> > >
> > >     file://~ Inner
> > > Classes ----------------------------------------------------------
> > >
> > >     private class CounterThread
> > >         extends Thread
> > >     {
> > >         file://~ Instance
> > > fields ----------------------------------------------------
> > >
> > >         private String name = "";
> > >
> > >         file://~
> > > Constructors -------------------------------------------------------
> > >
> > >         public CounterThread( String str )
> > >         {
> > >             super( str );
> > >             name = str;
> > >         }
> > >
> > >         file://~
> > > Methods ------------------------------------------------------------
> > >
> > >         public void run(  )
> > >         {
> > >             while ( true )
> > >             {
> > >                 try
> > >                 {
> > >                     count( name );
> > >                 }
> > >                 catch ( CreateException ce )
> > >                 {
> > >                     System.err.println( "Couldn't create" );
> > >                 }
> > >             }
> > >         }
> > >     }
> > > }
> > >
> > >
> > >
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